According to the Schiff theorem the nuclear electric dipole moment (EDM) is screened in neutral atoms. In ions this screening is incomplete. We extend a derivation of the Schiff theorem to ions and molecules. The finite nuclear size effects are considered including $Z^2{\alpha }^2$ corrections to the nuclear Schiff moment, which are significant in all atoms and molecules of experimental interest. We show that in a majority of ionized atoms the nuclear EDM contribution to the atomic EDM dominates, while in molecules the contribution of the Schiff moment dominates. We also consider the screening of electron EDM in ions.

• Received 20 December 2011

DOI:https://doi.org/10.1103/PhysRevA.85.022505